1. Field of the Invention
This invention relates to improvements in alloy steels. More particularly, it relates to steels for machine construction having excellent cold forgeability and machinability.
2. Description of the Prior Art
Most of machine parts have hitherto been fabricated by cutting operations. In order to reduce the cost of processing of the parts, there are frequently being adopted, with the recent advancement of cold work techniques, processes which include rough shaping by cold work and then finishing by cutting operation. To this end, there are required steels which have excellent characteristics both in cold forgeability and machinability.
In order to impart good cold forgeability to steels, it is the general practice that steels are subjected to the spheroidizing and annealing treatment or to a method of lowering cleanliness thereof. However, this leads to poor machinability of the steel. On the other hand, it is known to add S or Pb to steels so as to improve the machinability with the attendant disadvantage that the cold forgeability of steel becomes poor. As will be appreciated from the above, the cold forgeability and the machinability are, in most cases, contrary to each other and it is generally accepted that these properties are incompatible with each other.
In general, carbon steels or alloy steels for machine construction contain S in an amount of about 0.015-about 0.030%, most of the sulfur being present in steel in the form of MnS. The MnS is usually distributed unevenly in steel and tends to be excessively elongated upon rolling, causing the anisotropy of mechanical properties and deteriorating the cold forgeability (these inclusions in steel are defined as A-system inclusion in the JIS Standards). Al is ordinarily added for deoxidation or regulation of grain size into molten steel. Even after solidification of the molten steel, Al is contained in the form of Al.sub.2 O.sub.3 at a level of about 40-100 ppm and causes the development of incipient cracks within steel at the time of cold work, thus deteriorating the cold forgeability and accelerating the abrasion of tools at the time of cutting operations because of its high hardness (these inclusions are defined in JIS Standards as B-system inclusion).
Then, known Ca-deoxidized steel and lead free-cutting steels will be explained. The Ca-deoxidized steels are those which are obtained by deoxidizing molten steel with Ca to form Ca oxides such as anorthite (CaO.Al.sub.2 O.sub.3.2SiO.sub.2), gehlenite (2CaO.Al.sub.2 O.sub.3.SiO.sub.2) and the like, by which at the time of cutting, the deposit called belag is formed on the surfaces of tool to supress the tool abrasion. In some Ca-deoxidized steels, the Ca oxide may be CaO.nAl.sub.2 O.sub.3. The lead free-cutting steels comprises 0.15-0.30% of Pb which is present in the form of a single inclusion, or compound inclusions with sulfides or oxides. This Pb inclusion shows the effect of stress concentration and the lubricating action on tool at the time of cutting to improve the machinability. In this connection, however, ordinarily employed lead free-cutting steels, for example, S45CL, contain sulfur in an amount ranging 0.015 to 0.030%, most of which is present in the form of MnS. This MnS is elongated upon rolling and is turned as the A-system inclusion. Additionally, Pb is readily converted into a large-sized compound inclusion in combination with MnS, leading to the deterioration of cold forgeability.
The alloy steels of the type to which the present invention is directed are known, for example, in Japanese Patent Publication Nos. 46-27661 and 54-33206, Japanese Laid-open Application No. 53-22112, Great Britain Pat. Nos. 874,488 and 717,896 Norwegian Pat. No. 123,551, German Pat. (DE-OS) Nos. 1,036,297, 1,905,247 and 1,918,702, and U.S. Pat. No. 3,110,586 and 3,424,576.